[issue27533] release GIL in nt._isdir

[R. David Murray]

Changes by R. David Murray <rdmurray at bitdance.com>:


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components: +Windows
nosy: +paul.moore, steve.dower, tim.golden, zach.ware

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue27533>
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